1. Field of the Invention
The present invention generally relates to bipolar and field-effect transistor circuits, and more specifically to the compensation of inherent non-linearities in transistor devices to produce single-ended and differential transistor circuits with high linearity and low distortion.
2. Description of the Related Art
Bipolar and field-effect transistors have inherent non-linearities which limit their application. These non-linearities cause modulation of the output signal and distortion.
Negative feedback is commonly employed to increase the linearity of transistor amplifier circuits. However, this technique creates problems of high frequency distortion due to the finite bandwidth of closed loop feedback circuits.
Some of the non-linearities can be canceled through the use of complementary transistor circuits including transistors of both N and P conductivity types. However, complementary transistor circuits fabricated using currently available fabrication techniques are only capable of operation at low speeds. For this reason, prior art high speed transistor circuits include only resistors and transistors of a single conductivity type, usually N, and are vulnerable to non-linear signal modulation
Variation of the base-emitter current flow in a bipolar transistor causes a variation .DELTA.Vbe of the forward base-emitter voltage Vbe which in turn causes modulation or distortion of an output signal Vout. Vbe is the voltage drop across the base-emitter junction of the transistor Q1, and is generally on the order of 0.8 volts. Base-emitter current variation will result from the transistor driving a resistive or capacitive load, since the transistor has to source or sink the current flowing through the load.
This effect is known as "load current modulation", and has been compensated for in a common-emitter transistor amplifier as described in a textbook entitled "ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS", by P. Gray et al, John Wiley & Sons, 1977, pp. 566-570, and illustrated in FIG. 1. A main NPN type bipolar transistor Q1 is connected in a common-emitter configuration with its emitter grounded and its collector connected to a voltage source VCC through a load resistor R1. The output signal Vout is taken from the collector of the transistor Q1.
If an input signal Vin is applied directly to the base of the transistor Q1 and the voltage of the input signal Vin is increased, the collector current will increase by a non-linear (exponential) amount, resulting in a non-linear collector voltage. The operation is opposite for a decrease in the input signal Vin.
The non-linear modulating collector current is compensated for by an NPN type bipolar transistor Q2 having its emitter connected to a voltage source VEE through a resistor R2 and its collector connected to the base of the transistor Q1. The collector of the transistor Q2 is also connected to the cathode of a pre-distortion diode D1, the anode of which is connected to a reference voltage source VREF. The input signal Vin is applied to the base of the transistor Q2. The transistor Q1 and diode D1 are matched such that the junction of the diode D1 has the same voltage-current characteristic as the base-emitter junctions of the transistors Q1 and Q2.
Assuming that the resistance value RV1 of the resistor R2 is very large and the voltage of the input signal Vin is increased by .DELTA.Vin, the collector current of the transistor Q2 will increase by approximately .DELTA.I=.DELTA.Vin/RV1. The increased current flow through the diode D1 will cause the voltage across the diode D1 to increase by a non-linear (logarithmic) amount .DELTA.Vbe. This causes the voltage at the base of the transistor Q1 to decrease by .DELTA.Vbe. The reduced base voltage causes the base-emitter current of the transistor Q1 to decrease by a non-linear (exponential) amount.
The logarithmic and exponential transformations are mutually canceling, such that the decreased current flow through the base-emitter junction of the transistor Q1 causes the Vbe of Q1 to decrease by .DELTA.Vbe and the emitter current of the transistor Q1 to decrease by .DELTA.I. The .DELTA.Vbe of the diode D1 is equal and opposite to and cancels the .DELTA.Vbe of the transistor Q1, such that the .DELTA.Vbe of the transistor Q1 does not modulate the output signal Vout.
The arrangement illustrated in FIG. 1 is known in the art as "pre-distortion", since a modulation or distortion which is equal and opposite to the modulation effect in the main transistor is introduced into the signal flow upstream of the main transistor Q1. Although effective, the prior art arrangement of FIG. 1 is limited in that it does not compensate for other non-linear modulation effects.
The effective width of the base region in a bipolar transistor, or the length of the channel region in a field-effect transistor (FET), varies as a non-linear function of the collector-emitter voltage Vce (or the drain source voltage Vds in an FET). This causes modulation .DELTA.Vce of the collector-emitter voltage Vce of the transistor which distorts the output voltage Vout, and is known as "base-width modulation", "channel-width modulation" or the "Early effect".
Another source of signal modulation which is present in bipolar transistors, but not FETs, is due to the fact that the base current Ib is finite, and varies as a non-linear function of Vce to create a modulating current .DELTA.Ib. This is known as "alpha" error. The current gain can also vary as a non-linear function of collector current, causing modulation known as "current gain modulation" or "beta" error.